Document Type

Thesis - Open Access

Award Date

2025

Degree Name

Master of Science (MS)

Department / School

Mechanical Engineering

First Advisor

Jeffery Doom

Abstract

This thesis presents recommendations for optimizing hypersonic scramjets via a numerical study focusing on fuel type and cavity flameholder design and the impacts they have on supersonic combustion. The research is divided into two main investigations aimed at improving scramjet engine efficiency using two distinct geometrical configurations, a square cavity engine and an axisymmetric cavity engine. Each configuration is examined under Mach 2 inflow conditions to assess the role of geometric variation and fuel selection on combustion stability, flameholding, and overall performance. The first study focuses on a comparative assessment of hydrogen and ethylene as scramjet fuels to evaluate their effects on ignition characteristics, combustion efficiency, and emissions. Hydrogen demonstrated significantly enhanced flame stabilization and superior combustion efficiency, achieving a nearly twofold increase in specific impulse compared to ethylene while requiring only one-third of the fuel injection rate. Although peak flame temperature and total heat release were slightly reduced, overall performance improved due to faster ignition and a more favorable air–fuel ratio. Environmentally, hydrogen produced no carbon dioxide or soot and emitted only trace levels of NOₓ, positioning it as a cleaner and more sustainable fuel alternative for high-speed propulsion. The second study evaluates the influence of cavity length-to-depth ratios (L/D = 4, 6, 8, 10, and 12) within an ethylene-fueled, axisymmetric scramjet combustor. Simulations were conducted in STAR-CCM+ using the SST 𝑘-𝜔Reynolds-Averaged Navier–Stokes (RANS) turbulence model under a stagnation temperature of 600 K, stagnation pressure of 240 kPa, and a global equivalence ratio of 0.44. Results revealed that combustion was stably anchored near the cavity step, supported by strong stoichiometric regions that promoted ignition and sustained flame propagation. Temperature fields peaked within the cavity, while heat release was concentrated along the shear layer. The analysis further demonstrated consistent recirculation zones extending approximately two cavity depths downstream, with minor geometry-induced variations in wall pressure and shock-train positioning. Collectively, the findings emphasize the interdependence of cavity geometry and fuel selection in determining scramjet combustion behavior. Ethylene-based designs benefited from optimized L/D ratios that enhanced flow recirculation and flameholding, while hydrogen-based configurations achieved superior efficiency and reduced emissions without compromising stability. This integrated analysis underscores the importance of coupling geometric optimization with fuel selection to advance next-generation hypersonic propulsion systems. The insights gained provide a foundation for future scramjet development aimed at achieving both high performance and environmental sustainability in hypersonic flight.

Library of Congress Subject Headings

Airplanes -- Scramjet engines -- Combustion. 
Airplanes -- Scramjet engines -- Design and construction.
Jet planes -- Fuel. 
Jet propulsion.    
Aerodynamics, Supersonic.
Computational fluid dynamics. 
Flame stability.        

Publisher

South Dakota State University

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